Revisiting Asymptotic Theory for Principal Component Estimators of Approximate Factor Models

Abstract

It is well known that approximate factor models exhibit rotation indeterminacy. Principal component (PC) estimators are typically analyzed relative to a rotated factor-loading representation, but the commonly used rotation depends on the estimator itself, leaving unclear which fixed population parameters are estimated. We show that any starting representation of the common component can be mapped by a rotation matrix $\bH$, constructed without using the PC estimator or the idiosyncratic errors and unique up to column signs, to a population representation satisfying the same PC normalization as the estimator. Although $\bH$ depends on the starting representation, the resulting representation is invariant to that choice up to simultaneous column sign changes. We call the resulting factors and loadings the pseudo-true (PC-normalized) parameters. Under a general weak factor model allowing signal eigenvalues to diverge at possibly different rates, we establish consistency and asymptotic normality of the PC estimators for these fixed, estimator-independent targets, together with fixed-target asymptotic theory for factor-augmented regressions. The theory thereby justifies confidence intervals for PC-normalized factors, gives loading profiles a fixed-target interpretation, and identifies the population coefficients associated with estimated PC factors in factor-augmented regressions.